// 父背景 background: linear-gradient(to bottom right, #50A3A2, #53E3A6);
.bg-bubbles {
  position: absolute;
  // 使气泡背景充满整个屏幕；
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  // 如果元素内容超出给定的宽度和高度，overflow 属性可以确定是否显示滚动条等行为；
  overflow: hidden;
  li {
    position: absolute;
    // bottom 的设置是为了营造出气泡从页面底部冒出的效果；
    bottom: -160px;
    // 默认的气泡大小；
    width: 40px;
    height: 40px;
    background-color: rgba(255, 255, 255, 0.15);
    list-style: none;
    // 使用自定义动画使气泡渐现、上升和翻滚；
    animation: square 20s infinite;
    transition-timing-function: linear;
    // 分别设置每个气泡不同的位置、大小、透明度和速度，以显得有层次感；
    &:nth-child(1) {
      left: 10%;
    }
    &:nth-child(2) {
      left: 20%;
      width: 90px;
      height: 90px;
      animation-delay: 2s;
      animation-duration: 7s;
    }
    &:nth-child(3) {
      left: 25%;
      animation-delay: 4s;
    }
    &:nth-child(4) {
      left: 40%;
      width: 60px;
      height: 60px;
      animation-duration: 8s;
      background-color: rgba(255, 255, 255, 0.3);
    }
    &:nth-child(5) {
      left: 70%;
    }
    &:nth-child(6) {
      left: 80%;
      width: 120px;
      height: 120px;
      animation-delay: 3s;
      background-color: rgba(255, 255, 255, 0.2);
    }
    &:nth-child(7) {
      left: 32%;
      width: 160px;
      height: 160px;
      animation-delay: 2s;
    }
    &:nth-child(8) {
      left: 55%;
      width: 20px;
      height: 20px;
      animation-delay: 4s;
      animation-duration: 15s;
    }
    &:nth-child(9) {
      left: 25%;
      width: 10px;
      height: 10px;
      animation-delay: 2s;
      animation-duration: 12s;
      background-color: rgba(255, 255, 255, 0.3);
    }
    &:nth-child(10) {
      left: 85%;
      width: 160px;
      height: 160px;
      animation-delay: 5s;
    }
  }
}
// 自定义 square 动画；
@keyframes square {
  0% {
    opacity: 0.5;
    transform: translateY(0px) rotate(45deg);
  }
  25% {
    opacity: 0.75;
    transform: translateY(-400px) rotate(90deg)
  }
  50% {
    opacity: 1;
    transform: translateY(-600px) rotate(135deg);
  }
  75% {
    opacity: 0.75;
    transform: translateY(-1000px) rotate(180deg);
  }
  100% {
    opacity: 0;
    transform: translateY(-1400px) rotate(225deg);
  }
}
